This invention relates to methods for processing materials through the use of microwave energy.
The application of microwave energy to process various kinds of materials in an efficient, economic and effective manner is emerging as an innovative technology. Microwaves are electromagnetic radiation with wavelengths ranging from about 1 mm to 1 m in free space with frequencies between approximately 300 GHz to 300 MHz, respectively. Microwaves have a practical industrial range of between about 500 MHz and 10 GHz. Today, only narrow bands of frequencies centered at 915 MHz and at 2.45 GHz are permitted by regulation for industrial and scientific applications without a special license.
Microwave heating of materials is fundamentally different from conventional radiation, conduction or convection heating. In the microwave process, the heat is generated internally within the material instead of originating from external heating sources. The material of the article being processed in fact becomes the source of the heat during processing. Microwave heating is a sensitive function not only of the material of the article being processed but also depends on such factors as the size, geometry and mass of the article. Microwaves can be transmitted, absorbed or reflected, depending on the material type with which they interact.
Microwave heating has gained significantly over conventional heating in recent years for materials synthesis and sintering due to its rapid heating rates, reduced processing times, and substantial energy savings, as well as for being an environmentally cleaner technology. Often the use of microwave heating processes has resulted in products made of materials having better structural uniformity, finer microstructure and improved properties. In spite of these advantages, there are several problems which have hindered the use of microwaves as a unified tool for materials processing. For example, in the case of metals, it is well known that at room temperature, the presence of solid or monolithic metal parts or thick metal films in a microwave field results in large electric field gradients which often cause visible and sometimes strong electric discharges. It was recently discovered that powder metal articles can be sintered by subjecting them to microwave energy to form a dense metal part. See U.S. Pat. No. 6,183,689 B1, issued Feb. 6, 2000, and entitled Process for Sintering Powder Metal Components. However, it is generally believed that monolithic metal articles at room temperature invariably cause plasma sparking within a microwave cavity, and they are therefore unsuitable for microwave processing, unless the metals have been previously, conventionally heated above a critical temperature, which is normally between about 400xc2x0 C. and 600xc2x0 C., above which they absorb microwaves and can be processed using microwaves without sparking.
In the case of ceramics, many ceramic materials do not couple well with microwave radiation at low temperatures. Since the use of microwaves for material synthesis or sintering rests heavily on the microwave absorbing capacity of the material being processed, these ceramic materials also have to be preheated by another heating source. One preheating source which has been used is a secondary microwave susceptor such as a bed of certain susceptor materials packed around the ceramic materials as the ceramics are being sintered or otherwise processed in a microwave field. The use of packed susceptor beds has often resulted in uneven heating. Additionally, as the ceramic material is sintered, it may shrink and lose contact with the bed, decreasing the effectiveness of the susceptor bed. The materials used in some microwave susceptor beds may themselves sinter or fuse together in the bed, leading to uneven or insufficient sintering of the material being heated. Additionally, some secondary microwave susceptor materials may decompose, contaminate or react with the material being processed. It has been reported that the use of carbon alone as a susceptor bed, as well as the use of related materials such as graphite and SiC, with or without placing an item being sintered in a graphite crucible, limits the maximum temperature that can be achieved to values much less than those required for sintering. See Example 9, Col. 11, line 59-Col. 12, line 12 of U.S. Pat. No. 5,808,282 issued Sep. 15, 1998.
Alternately, rods made of SiC or MoSi2 or similar materials are placed adjacent to the ceramic material being processed as secondary microwave susceptors to heat the ceramic material at low temperatures in the microwave field. The use of such rods as microwave susceptors has met with limited success due to slow heating rates encountered and the high cost of the rods themselves.
Additionally, the processing of other materials which are heated with microwaves, such as powdered metals, composites and glass, can benefit from an improved microwave process that decreases the required heating time and improves the properties of the material.
In accordance with one embodiment of this invention, a method of heating an article being processed with microwave energy comprises providing a thin layer of highly microwave absorbent powdered material around at least a portion of a container made of microwave transmitting material for the article. The article is placed within the container at a position where the article is adjacent the thin layer of highly microwave absorbent powdered material. Microwave energy is applied to the container to heat the article.
The highly microwave absorbent powdered material (xe2x80x9cHMAPMxe2x80x9d) used in this method comprises any material which can, at room temperature, absorb most or substantially all of the microwaves directed at it and generate heat which can be transferred to the article being heated. HMAPM includes powdered material having a high percentage by weight of carbon, such as carbon, graphite, carbon black, SiC and coal.
The thin layer of HMAPM can be provided around at least a portion of a container in any convenient manner to initially generate heat for the article being processed. The thickness of the layer of HMAPM is determined empirically. In one embodiment, the thin layer is formed by mixing HMAPM with a liquid to make a paste. Water is preferably used, but any other satisfactory liquid may be used, as well. The paste can be applied as a thin layer to the outside of the container and dried or allowed to dry before use in a microwave oven. In another embodiment, HMAPM in a dry, powdered state can be placed between the walls of a double-walled container. Or a double-walled container may have HMAPM applied to one or both of the inside surfaces of the double walls and dried so that the dried paste forms a layer of HMAPM between the walls. The thin layer of HMAPM between the double walls in either a powder or paste form may be vacuum sealed or sealed with an inert gas. Alternatively, the thin layer of HMAPM may be retained on or within a structure located around at least a portion of the container. Examples of such structures are spirals or one or more rings.
An article is adjacent the thin layer of HMAPM in the container when it is in the vicinity of the thin layer so that the article obtains the benefit of the absorption of the microwave energy by the thin layer of HMAPM and receives the heat transfer from the thin layer. How close any particular article should be to the thin layer is determined empirically and will depend on the size and shape of the article being heated, the material out of which the article is constructed, and the size and shape of the container in which the article is placed.
While the method of this invention may be used in a number of different embodiments, it is particularly useful in an embodiment in which the thin layer of HMAPM is provided around the total circumference of the container with a length at least equal to the length of the article being heated. Making the coating somewhat longer than the length of the article can assure even heating of the article. The article is placed within the length of the thin layer of HMAPM and microwave energy is applied to the container to heat the article.
Alternatively, uneven heating can be provided to articles, such as composites of two or more materials, by either providing a thin layer of HMAPM around less than the total circumference of the container, and placing the part of the article which is to receive greater heat adjacent the thin layer, or providing a thin layer of HMAPM on one side of a sheet of material or between the walls of a double-walled sheet of material and placing the part of the article to receive greater heat adjacent the thin coating. Alternatively, uneven heating of an article can be accomplished by placing less than the full length of the article adjacent the thin coating. When an article is properly placed with respect to the thin layer, microwave energy is applied to the container or to the sheet of material, as the case may be.
Articles to be heated according to this method may be made of industrial materials which can be processed with microwaves, including ceramics, powdered metals and glass. Where the thin layer of HMAPM is provided around the total circumference of a container and the article can be placed within the length of the thin layer, this method can be used to heat articles made of monolithic metals, including Cu., Ag., Au., Pt., Al., Ga., Fe., steel, brass, W., Re., Ta., Mo., Nb., Ni., Co., V., Cr., Sn., and metal alloys and metal compositions of any of the foregoing metals, regardless of whether the metals are at room temperature or any other temperature.
This invention does not reside in any one of the features of the method disclosed above or more fully described in the Detailed Description and claimed below. Rather, this invention is distinguished from the prior art by the combination of features which make it a unique method. Important features of this invention are shown and described below to illustrate the best mode contemplated to date of carrying out this invention.
However, those skilled in the art will understand that this invention is capable of being carried out in many embodiments which are different from those shown and described herein. Thus, the details of the method of this invention can be changed in various manners without departing from the scope of this invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and are not to restrict the scope of this invention. Additionally, the claims are to be regarded as including such equivalent methods as do not depart from the nature and scope of this invention.